1. Field of the Invention
The present invention relates to a receiving apparatus for a multiple input multiple output (MIMO) system and a method thereof; and, more particularly, to a receiving apparatus and a method thereof for improving a signal detection performance with computational complexity reduced using linear and nonlinear receiving schemes together when a receiving signal is detected in a double space time block code-orthogonal frequency division multiplexing (STBC-OFDM) system which is one of MIMO-OFDM technologies.
This work was supported by the IT R&D program of MIC/IITA [2006-S-014-02, “Development of IEEE 802.11n Modem & RF Chip-sets with Data Rate 200 Mbps”].
2. Description of Related Art
It is a requirement of a wireless communication system to transmit a large amount of high quality multimedia data using a limited frequency. As a method for transmitting a large amount of data using a limited frequency, a multiple input and multiple output (MIMO) system was introduced. The MIMO system forms a plural of independent fading channels using a multiple antenna at receiving and transmitting ends and transmits different signals through each of transmitting antennas, thereby significantly increasing a data transmit rate. Accordingly, the MIMO system can transmit a large amount of data without expanding frequency.
However, the MIMO system has a shortcoming that the MIMO system is too fragile for inter-symbol interference (ISI) and frequency selective fading. In order to overcome the shortcomings, an orthogonal frequency division multiplexing (OFDM) technology was used. The OFDM scheme is the most proper modulation scheme for transmitting data at a high speed. The OFDM scheme transmits one data row through a subcarrier having a low data transmit rate.
A channel environment for wireless communication has a multiple paths due to obstacles such as buildings. In a wireless channel environment having multi-paths, delay spray is generated due to the multiple paths. If a time of delay spray is longer than a time of transmitting a next symbol, inter-symbol interference (ISI) is generated. In this case, fading is selectively generated in a frequency domain (frequency selective fading). In case of using single carrier, an equalizer is used to remove the ISI. However, complexity of the equalizer increases if a data transmit rate increases.
The shortcomings of the MIMO system can be attenuated using an orthogonal frequency division multiplexing (OFDM) technology. In order to overcome the shortcomings of the MIMO system with the advantages of the MIMO system maintained, an OFDM technology was applied to a MIMO system having N transmitting antennas and N receiving antennas. That is, a MIMO-OFDM system was introduced.
FIGS. 1a and 1b are block diagrams schematically illustrating a double STBC-OFDM system, which is one of MIMO-OFDM technologies. FIG. 1a is a block diagram illustrating a transmitting apparatus, and FIG. 1b is a block diagram illustrating a receiving apparatus.
Referring to FIG. 1a, the transmitting apparatus of the double STBC-OFDM system includes a demultiplexer 101, encoders 102, interleavers 103, quadrature amplitude modulation (QAM) mappers 104, inverse fourier transform (IFFT) units 105, cyclic prefix (CP) inserters 106, serial-to-parallel (S/P) converters 107, and space time block code (STBC) transmitters 108. The demultiplexer 101 divides a transmit bit stream into a plurality of data rows. The encoders 102 encode corresponding data rows. The encoded data are interleaved by the interleavers 103 and inputted to the QAM mappers 104. The QAM mappers 104 modulate the interleaved data according to a modulation scheme such as binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (16QAM), and 64QAM. The modulated symbols are transformed to signals in a time domain through inverse fast Fourier transform (IFFT) units 105. The CP inserters 106 insert a cyclic prefix (CP) code for a guard interval into the transformed symbols. The S/P converters 107 convert the CP code inserted signal to parallel signals. The STBC transmitter 108 transmits the parallel signals through a wireless channel.
Referring to FIG. 1b, the receiving apparatus of the double STBC-OFDM system includes CP removers 109, FFT units 110, a MIMO receiver 111, a decoder 112, an interleaver 113, and a mapper 114. The CP removers 109 remove CP codes for a guard interval and transfer the CP code removed signal to the FFT unit 110. The FFT unit 110 performs FFT on the input parallel signal. The MIMO receiver 111 estimates a transmitting data symbol generated through FFT. The MIMO receiver 111 calculates a log likelihood ratio (LLR) from the estimated symbol. The decoder 112 decodes each of data rows transferred from the MIMO receiver 110 and estimates transmission data.
The receiving apparatus includes an interleaver 113 and a mapper 114 for the present invention. The interleaver 113 and the mapper 114 will be described with reference to FIG. 2 in later.
As described above, the STBC-OFDM technology was developed by applying a typical transmit diversity technology to an OFDM technology. The STBC-OFDM technology performs space-time coding in a time domain. The STBC-OFDM technology may improve a link level performance by providing the transmit diversity. However, the STBC-OFDM technology cannot improve the link level performance in a channel that varies with a time. That is, the orthogonality of a signal is broken because an assumed state of a static channel in time-space coding is not guaranteed in a channel that varies with a time. Therefore, the STBC-OFDM technology is applied only to a wideband system such as a wireless LAN system that occupies a wideband width and uses a short packet.
A double STBC-OFDM system was developed by applying a spatial multiplexing technology for transmitting data in parallel through a wireless channel to a typical STBC-OFDM system. The double STBC-OFDM system has a data transmit rate two times higher than that of a typical STBC-OFDM system.
Two types of receivers were used as a receiver for a typical STBC-OFDM system, a nonlinear type receiver using maximum likelihood (ML) detection and a linear type receiver using zero forcing (ZF). The nonlinear type receiver has a shortcoming of high complexity although it has excellent receiving performance. Therefore, it is difficult to realize the nonlinear type receiver. On the contrary, the linear type receiver has an advantage of low complexity although it has comparatively low receiving performance. Due to such an advantage of the linear type receiver, the linear type receiver has been generally used as a detector of a MIMO system.